Special Weather Statement, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
305 PM CDT Sat May 26 2018

.DISCUSSION.../Tonight through Saturday/
Issued at 300 PM CDT Sat May 26 2018

Only concern through the rest of today is isolated storms east of
I-35 this afternoon and early evening. The development of this
convection has been poorly handled by most hi-res model solutions
today. Surface convergence is very weak, but a passing wave noted
on water vapor is likely aiding some surface based parcels to
breach the LFC. Any convection should push east of the forecast
area by early evening. Weak shear will limit the threat of
organized strong updrafts, but local HRRR/RAP soundings indicate
1000-1500 J/kg of DCAPE and modest lapse rates. Thus can't rule
out the potential for a few strong wind gusts and some hail.

Not much change in the short term forecast as confidence remains
high for record breaking heat tomorrow and possibly again on
Monday. Stagnant upper level pattern will keep an amplified ridge
axis parked over the region for the next couple days, downstream
of a large area of low pressure out west. Both NAEFS and Euro
ensembles show 500mb heights exceeding climatological max over
the state tomorrow. Subtle warm air advection in the low to mid
levels overnight will lead to a slight bump in highs tomorrow
with most of central Iowa in the mid to upper 90s. Diurnal mixing
once again transports some drier air from just off the surface
into the boundary layer and holds heat index values just below
the triple digits. Not much change on Monday as the ridge axis
barely nudges east. Increasing mid-level moisture may promote
more widespread diurnal CU develop over the south and west and
hold highs back a couple degrees. Otherwise it will be dry and
quiet as deep layer subsidence/Q-vector divergence beneath the
ridge suppresses thunderstorm potential.

Ridge axis finally pushes east on Tuesday as the western conus low
emerges into the eastern Rockies. A lead shortwave in the
southwest flow aloft may trigger some showers and storms in our
western counties, but the higher chances for deep convection will
likely reside in the vicinity of a slow moving front in NE/SD.
Local precip chances increase on Wednesday as the main upper level
low forces the front through the state. It should be noted that
forecast confidence is quite low due to the phasing the post-
tropical remnants of Alberto into the upper level flow, a process
the models will likely struggle with over the coming days. Right
now all models keep any moisture associated with Alberto east of
our area, but it will still impact the evolution of the upper
level low crossing the northern Plains and associated precip
chances. Should thunderstorms materialize in central Iowa on
Wednesday the threat for severe weather will be limited by rather
weak flow aloft.

Model guidance favors above normal temperatures through the end of
the week as a ridge quickly re-establishes over the Plains
states. Thunderstorm chances hold off until next weekend as
as the next upper level low emerges from the Rockies.

&&

.AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/
Issued at 1251 PM CDT Sat May 26 2018

VFR conditions expected throughout the period. Some daytime
cumulus development this afternoon, however will remain scattered
in nature and will not impact flight categories. Winds will
remain light, switching to southerly overnight.

&&

.CLIMATE...
Issued at 300 PM CDT Sat May 26 2018

.Forecast/Record Highs for Official Central Iowa Climate Sites...

====================================================================
            |        Sunday        |        Monday        |  Period
            |  Forecast   Record/  |  Forecast   Record/  |    of
City        |    High      Year    |    High      Year    |  Record
====================================================================
Des Moines        97     91/2012         93      92/1895      1878-
Waterloo          96     93/1900         95      93/2006      1895-
====================================================================

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

DISCUSSION...Martin
AVIATION...Hagenhoff
CLIMATE...Martin

Hazardous Weather Outlook
National Weather Service Des Moines IA
308 PM CDT Sat May 26 2018

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-272015-
Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo-
Pocahontas-Humboldt-Wright-Franklin-Butler-Bremer-Sac-Calhoun-
Webster-Hamilton-Hardin-Grundy-Black Hawk-Crawford-Carroll-Greene-
Boone-Story-Marshall-Tama-Audubon-Guthrie-Dallas-Polk-Jasper-
Poweshiek-Cass-Adair-Madison-Warren-Marion-Mahaska-Adams-Union-
Clarke-Lucas-Monroe-Wapello-Taylor-Ringgold-Decatur-Wayne-
Appanoose-Davis-
308 PM CDT Sat May 26 2018

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

Hot conditions will continue into the early evening hours with
maximum heat index values in the 90s. Isolated thunderstorms are
also possible east of I-35 through early evening. The threat for
severe weather is low, but a few strong wind gusts and small hail
are possible.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

Unseasonably hot and somewhat humid conditions are expected to
continue through the Memorial Day Weekend into early next week
with maximum heat index values in the 90s. With this being the
first significant heatwave of the season, this could lead to heat
related impacts if precautions are not taken.

Thunderstorms are forecast Tuesday into Wednesday. The threat for
severe weather is low at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be needed.

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